compiler: Revert support for None as TNone

This was mistakenly included in fb2b634c4a, and broke the test
case verifying that using None as an ARTIQ type annotation in fact
generates an error message.

artiq/compiler/embedding.py

@@ -950,9 +950,6 @@ class Stitcher:
         return function_node
 
     def _extract_annot(self, function, annot, kind, call_loc, fn_kind):
-        if annot is None:
-            annot = builtins.TNone()
-
         if not isinstance(annot, types.Type):
             diag = diagnostic.Diagnostic("error",
                 "type annotation for {kind}, '{annot}', is not an ARTIQ type",